* {
  margin: 0;
  padding: 0;
}

.main {
  width: 550px;
  height: 150px;
}

#smallBox {
  float: left;
  width: 300px;
  height: 150px;
}

#smallBox img {
  width: 300px;
  height: 150px;
  display: block;
  position: relative;
}

#mask {
  width: 50px;
  height: 50px;
  background-color: rgba(0, 0, 0, .5);
  position: absolute;
  top: 0;
  left: 0;
  cursor: pointer;
  z-index: 999;
  display: none;
}

#largeBox {
  float: left;
  margin-left: 50px;
  width: 200px;
  height: 150px;
  overflow: hidden;
  position: relative;
  display: none;
}

#largeBox img {
  width: 1200px;
  height: 600px;
  display: block;
  position: absolute;
  top: 0;
  left: 0;
}